Quantitation of protactinium, 231Pa in abyssal carbonates at 10−12 g g−1 levels, presents certain challenges because it is the only isotope of this element that is present in the natural environment and its half-life of 32800 y is too long for useful measurement using alpha-particle or gamma-ray spectroscopy. However, because it is an isotope in the 235U decay chain, closely linked to the 238U decay chain, quantitation can be achieved relative to a carbonate standard in secular radioactive equilibrium. The 238U, 235U, 234U, 230Th and 232Th isotopes in the carbonate standard are determined with a mixed 236U–229Th spike using isotope dilution protocols and a Multi-Collector Inductively Coupled Plasma Mass Spectrometer.